Stats don't lie. I only just started playing online MP on the Ecks-Bawks Free-Sixty two weeks ago. I've only played Halo 3 and MW2 so far. Both games have stats I've noted to different degrees.
In Halo 3, the stats are quite clear: according to Waypoint I'm almost exactly the average Social gamer (1.01 K/D ratio and 0.75 Ranked). Of course, this doesn't tell the whole story. I've suffered many a class A whooping in Social games to those who by rights shouldn't have even been in the Social room - guys who were clearly a well-oiled team/murder-machine. I've also felt the sting of frequently partying up with my friends, most of whom really shouldn't be online in pure skill terms. This often leads to hideously unbalanced teams when the matchmaking system (thankfully, but still painfully) keeps us all together against a team full of generals, colonels and so forth. What's more, I find my K/D ratio is extremely game-type sensitive. In Team Swat I range from the upper middle of the pack to the scoring leader, whereas in big team matches with vehicles and wide open maps I often flounder; I have neither the ability nor inclination to pick up the biggest baddest weapon/vehicle and spam it. This leads to many Spazer/Rocket/sniper/vehicle deaths.
In MW2, the story is very different. I'm quite clearly above average. In fact, in my third ever online game I took the highest individual score by a wide margin. This probably doesn't say much though - most of the players are utter shite. When I can win ~70% of the head-to-head I-know-that-you-know-that-I-know duels it's a sign the player base just isn't that good.
